The pubertal process is marked by significant adrenal and lymphoid tissue development and social behavior development. Because the adrenal and immune systems are highly responsive to environmental conditions, social experiences during this period may have a significant impact on the development of these two systems. I will test the hypothesis that puberty is a sensitive period during which individual interactions with the social environment produce life-long changes in adrenal and immune function and disease risk. Recent data from rats and children indicate that during peri-puberty, adrenal and immune functioning begins to differ significantly among individuals, with some individuals developing response patterns similar to those associated with depression (e.g. glucocorticoid hypersecretion and compromised immunity), which is often first diagnosed shortly after puberty. I propose a study with rats that will identify the peri-pubertal social conditions and personality traits that predict glucocorticoid hypersecretion and immune suppression in adult rats. Experiments involve manipulations of peri-pubertal social environment relative to personality traits to determine what combination of these two variables leads to differential adrenal and immune function in adulthood.